1. Field of the Invention
The present invention relates to a group III-V compound semiconductor represented by the general formula InGaAlN and a method for producing the same.
2. Description of the Related Art
A group III-V compound semiconductor represented by the general formula InxGayAlzN, where x+y+z=1, 0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61, and 0xe2x89xa6zxe2x89xa61 can be utilized as a material for highly efficient light-emitting device ranging from an ultraviolet to visible ray region because they allow direct band gap energy to be adjusted to optical energy having a wavelength in a region ranging from ultraviolet to red by changing the composition of the group III elements. Further, such a group III-V compound semiconductor has a larger band gap than conventionally-used typical semiconductors such as Si and GaAs and, hence, keeps its semiconductor properties even at such an elevated temperature at which the conventional semiconductors cannot operate. For this reason, the group III-V compound semiconductors principally allow the fabrication of electronic devices having superior environmental resistances.
Such a compound semiconductor is difficult to grow into a large crystal because of its very high vapor pressure at around its melting point. For this reason, there has so far not been obtained a large crystal of such a compound semiconductor for practical use as a substrate to be used in the fabrication of a semiconductor device. Thus, a compound semiconductor of this type, in general, is produced by allowing the compound semiconductor to epitaxially grow on a substrate of a material, such as sapphire or SiC, that is similar in crystal structure to the compound semiconductor and capable of providing a large crystal. At present, a relatively high-quality crystal of the compound semiconductor can be obtained by this method. Even in this case, it is difficult to reduce crystalline defects resulting from the difference in lattice constants or expansion coefficients between the substrate material and the compound semiconductor and the resulting compound semiconductor generally has a defect density of about 108 cmxe2x88x922 or more.
A technique was reported of obtaining such a group III-V compound semiconductor having a reduced defect density from a compound semiconductor having a high density of crystalline defects (Jpn. J. Appl. Phys., vol. 36, p. L899, 1997). According to this report, the aforesaid compound semiconductor having a high defect density (hereinafter may be referred to as xe2x80x9cunderlying crystalxe2x80x9d) is covered with an SiO2 pattern while fine openings are left uncovered, and a crystal growth on this structure is performed again (such a second or subsequent crystal growth may hereinafter be referred to as xe2x80x9cre-growthxe2x80x9d).
In the initial stage of re-growth, what is called xe2x80x9cselective growthxe2x80x9d occurs such that crystal growth does not occur on the pattern but only through the openings. As the growth continues further from this stage, crystal grown through the openings begins to extend over the pattern, and after a while, results to form a structure in which the pattern is buried. Immediately after the pattern has been buried, the crystal grown in the re-growth process has an uneven surface. However, the unevenness of the surface of the crystal is reduced as the crystal growth proceeds further, and finally, a crystal having a flat surface can be obtained.
Such a buried structure is confirmed to have a significantly lowered dislocation density in the re-grown layer. However, mechanisms of reducing defects differ depending upon the growing technique or growing conditions. Such mechanisms can be roughly divided into the following two sorts. The first mechanism is such that the re-grown layer takes over threading dislocations from the underlying substrate but the pattern terminates such threading dislocations thereunder and, hence, portions of the re-grown layer on the pattern have no dislocation, as shown in FIG. 1. In this case, however, dislocations can be reduced only in the portions of the re-grown layer lying on the pattern and reduction of dislocations can hardly be expected in portions of the re-grown layer that lying above the openings because such portions take over dislocations from the underlayer.
The second mechanism is such that portions of the re-grown layer above the openings grow with forming facets and the direction of each threading dislocation taken over from the underlayer are bent into the growing plane by the facets with the result that the defect density is reduced as the thickness of the growing layer increases, as shown in FIG. 2. Contrary to the structure resulting from the first mechanism, the structure in this case has dislocations concentrated in portions on the pattern but a lowered dislocation density in portions above the openings. To reduce these dislocations sufficiently, the re-grown layer is required to grow to a thickness as thick as several ten xcexcm or more. Such a thick film causes a resulting substrate to bow to such an extent that cannot be negligible. This causes sometimes problems that temperature distribution along the substrate plane becomes large when further growth is carried out on the substrate, and that a later device fabrication process cannot be achieved with a desired precision because of the unevenness of the substrate.
An object of the present invention is to provide a method of producing a group III-V compound semiconductor having a low dislocation density without increasing the thickness of a re-grown layer, the method including a re-growing process using a mask pattern, and to provide such a group III-V compound semiconductor having a low dislocation density.
As a result of extensive studies in view of the above situations, the present inventors have found that the above problems can be avoided by employing a re-growing process which provides a specified structure between the pattern and the crystal grown on the pattern.
That is, the present invention is directed to item (1): a group III-V compound semiconductor comprising: a layer of a first group III-V compound semiconductor represented by the general formula InuGavAlwN, where 0xe2x89xa6uxe2x89xa61, 0xe2x89xa6vxe2x89xa61, 0xe2x89xa6wxe2x89xa61, and u+v+w=1; a pattern formed on the layer of the first group III-V compound semiconductor and made of a material different from the first group III-V compound semiconductor and from a second group III-V compound semiconductor below; and a layer of the second group III-V compound semiconductor represented by the general formula Inx,GayAlzN where 0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61, 0xe2x89xa6zxe2x89xa61, and x+y+z=1, said layer is grown through the openings of the pattern; wherein voids are formed on the pattern, and threading dislocations in the layer of the second compound semiconductor are terminated by the voids.
The present invention is also directed to item (2): a method of producing a group III-V compound semiconductor recited in item (1), comprising a process for forming a layer of the second group III-V compound semiconductor, the process includes: a first step of growing the layer of the second group III-V compound semiconductor through the openings of the pattern until surface portion thereof parallel to the surface of the layer of the first group III-V compound semiconductor disappears; and a second step of continuing the growth of the layer of the second group III-V compound semiconductor with changing the growing condition so as to allow to have surfaces parallel to the surface of the layer of the first group III-V compound semiconductor, thereby forming voids on the pattern.
The present invention is also directed to item (3): a method of producing a group III-V compound semiconductor recited in item (1), comprising a process for forming a layer of the second group III-V compound semiconductor, the process including: a first step of growing the layer of the second group III-V compound semiconductor through the openings of the pattern until surface portion thereof parallel to the surface of the layer of the first group III-V compound semiconductor disappears; and a second step of continuing the growth of the layer of the second group III-V compound semiconductor with changing the growth condition so as to allow the layer to have surfaces of (0001) plane and facets perpendicular to the (0001) plane, thereby forming voids on the pattern.
The present invention is also directed to item (4): a method of producing a group III-V compound semiconductor recited in item (1), comprising a process for forming a layer of the second group III-V compound semiconductor, the process includes: a first step of growing the layer of the second group III-V compound semiconductor through the openings of the pattern until surface portion thereof parallel to the surface of the layer of the first group III-V compound semiconductor disappears; a second step of continuing the growth of the layer of the second group III-V compound semiconductor so as to have slanted facets only; and a third step of continuing the growth of the layer with changing the growth condition so as to allow the layer of the second group III-V compound semiconductor to have surfaces parallel to the surface of the layer of the first group III-V compound semiconductor, thereby forming voids on the pattern.
The present invention is also directed to item (5): a method of producing a group III-V compound semiconductor recited in item (1), comprising a process for forming a layer of a second group III-V compound semiconductor, the process includes: a first step of growing the layer of the second group III-V compound semiconductor through the openings of the pattern until a surface portion thereof parallel to the surface of the layer of the first group III-V compound semiconductor disappears; a second step of continuing the growth of the layer of the second group III-V compound semiconductor so as to have slanted facets only; and a third step of continuing the growth of the layer with changing the growth condition so as to allow the layer of the second group III-V compound semiconductor to have surfaces of (0001) plane and facets perpendicular to the (0001) plane, thereby forming voids on the pattern.
The present invention is still also directed to item (6): a group III-V compound semiconductor obtained by performing at least once a process including forming a pattern and re-growing a group III-V compound semiconductor on a layer of the group III-V compound semiconductor of item (1).